The present invention relates to a process for producing metallic salt of ethercarboxylic acid (hereinafter referred to as "ethercarboxylic product") useful as an organic chelating agent. More particularly, the present invention relates to the preparation of ethercarboxylic product in a highly pure form by restraining the formation of fumaric acid likely to occur in the manufacture of the product when maleic acid reacts with oxycarboxylic acid.
Ethercarboxylic products are known for their strong chelating ability by which metallic ions are confined in rings; particularly tetrasodium tartrate monosuccinate (I) and hexasodium tartrate disuccinate (II) are known as useful detergent builders, the formulas of these two salts are respectively as follows: ##STR1##
The production of metallic salt of ethercarboxylic acid is suggested in many literatures. U.S. Pat. Nos. 4,058,560, 4,188,493 and British Patent No. 1,389,732 disclose processes for manufacturing salts of carboxymethyloxy succinic acids. EP No. 236,007 discloses the preparation of salts of oxydisuccinic acid, and U.S. Pat. No. 4,663,071 and EP No. 266,723 disclose the preparation of salts of tartaric monosuccinic acids and tartaric disuccinic acids with the use of maleic acid and tartaric acid, and the adaptation of them for detergent builders. In common with these known processes, oxycarboxylic acids such as glycolic acid, malic acid, tartaric acid or tartronic acid, and maleic acid are reacted in the presence of zinc and/or alkaline earth metal, to which alkali metal hydroxide is added until the pH value is raised to 10 to 13. The reaction takes place at a concentration of 20 to 60 wt % organic acid salt, at a temperature of 50.degree. to 120.degree. C. over a period of 0.5 to 10 hours. In this way metallic salt of ethercarboxylic acid is obtained. Then alkali metal carbonate is added to remove the zinc and/or alkaline earth metal as zinc carbonate and/or alkaline earth metallic carbonate. Being free from the zinc and/or alkaline earth metal, the alkali metal salt of ethercarboxylic acid becomes useful as a detergent builder.
Under the known processes mentioned above, insoluble organic acid salts are produced in the reaction between the carboxylic acids and the metals. On the other hand, as the concentration of these organic acid salts becomes high, the ethercarboxylic product reaches a state of equilibrium for a shorter period of time, thereby securing a high yield of the product. However, if the industrial-scale process is carried out at such a high concentration from the start to the end, the viscosity becomes high because of a large amount of insoluble substances, thereby making it difficult to stir the mixture. In addition, heat is generated by neutralization between the acid and the base. There arises a need for dissipating the heat. In this way the process encounters with the difficulty in stirring and the necessity of dissipating the heat, which obstructs the smooth prosecution of the process. The known processe is carried out at a reduced concentration so as to secure the smooth stirring and quick heat dissipation even though the yield is sacrificed.
Commercial oxycarboxylic acids such as malic acid, glycolic acid and tartaric acid are generally on sale in solid. To put them in industrial use, they are diluted with water before starting an addition reaction. However oxycarboxylic acids are generally manufactured in aqueous solution. If they are used in aqueous solution, it will be unnecessary to concentrate the aqueous solution and then dry into solids.
One proposal is to supply the material in the form of a slurry, and to concentrate it to the extent that the stirring and heat dissipation are not prevented. Then the addition reaction is carried out. However, it has been found that the concentrating of the slurry involves the formation of fumaric acid, which is difficult to react with oxycarboxylic acids. This leads to the low yield of ethercarboxylic products, and also makes it difficult to obtain them in a pure state. There seems to be at least two reasons for the formation of fumaric acid; one is that maleic acid is subjected to cis-trans isomerism and transform into fumaric acid, and the other is that the metallic salts of ethercarboxylic acid is subjected to a reversible reaction, thereby decomposing into the original oxycarboxylic acid and maleic acid. About 50% of unsaturated acids becomes fumaric acid while the decomposition proceeds.
In the addition reaction mentioned above zinc and/or alkaline earth metal are normally added, but these substances produce zinc salt of ethercarboxylic acid and/or alkaline earth metallic salt. However, alkali metal salts of ethercarboxylic acid are known as useful detergent builders, but zinc salt and/or alkaline earth metallic salt of ethercarboxylic acid are not useful as detergent builders. On the other hand, all these metallic salts are useful as chelating agents, which captures heavy metals. As a result, the present invention aims at utilizing the chelating ability of alkali metal salt, zinc salt of ethercarboxylic acid and/or alkaline earth metallic salts.
The ethercarboxylic products find many applications, and as an example the manner of using it as a detergent builder will be described:
As described above, zinc salts and/or alkaline earth metallic salts are not useful as detergent builder. To remove such useless substances a special process is required. However, if zinc and alkaline earth metals are removed in the form of salts, it leads to the low yield of ethercarboxylic product. As a result, it is necessary to substitute zinc salt and/or alkaline earth metal with alkali metal. To substitute these metals, alkali metal carbonate and/or bicarbonate is added to zinc salt and/or alkaline earth metallic salt of ether carboxylic acid. The zinc carbonate and/or alkaline earth metallic carbonates are removed as sediments by decanting or any other similar method. However, the removal of these useless substances unavoidably involves the discharge of ethercarboxylic acid in the form of alkali metal salt. This not only leads to the low yield of ethercarboxylic product but also to pollution problems and waste of resources.